Heat a griddle or cast iron pan to high heat. Place tomatoes and serrano peppers on griddle until they become soft and slightly blackened, about 5 min.
Flip peppers, tomatoes, and continue to char for another 5 minutes.
Remove ingredients from griddle and set aside.
Start grinding the peppers in the molcajete one at a time. Once the desired consistency is reached, add the tomatoes one by one and keep grinding. Set aside
If you do not have a molcajete you can use a blender or food processor. Simply place the charred tomatoes and peppers in the machine and pulse until the desired consistency is reached. Set aside.
Add radish, watercress, and spring onion to the tomato and serrano salsa. Season with salt and pepper.
Serve with avocado slices and warm tortillas.
Yields about 2 cups. Mexican spring onions are very different from scallions. They have a bulbous ending and a very long green stem. See ingredient picture above. Feel free to add more serranos if you want this to be spicier.